1. Field of the Invention
The present invention relates to micro electromechanical systems (MEMS) and production methods thereof, and more particularly to vertically integrated MEMS systems.
2. Description of the Prior Art
The field of MEMS has entered a rapid growth phase with an explosion of applications for MEMS-based sensors. While the attention of the popular press has highlighted genomics labs on a chip and microscopic robots, the biggest advantage of MEMS technology may not come from making tools and sensors smaller. It will be from making them cheaper. Yet, most MEMS devices are still stand-alone components and the cost advantages of ubiquitous integration are lost with this approach. MEMS devices are conventionally manufactured using the methods adapted from making electronic microchips. Integration enabled the microelectronics revolution. Large Scale Integration (LSI) put millions of transistors onto a single chip, while continually bringing more power to the marketplace at steadily decreasing costs. Microelectronics has moved to the next level of integration, making systems on a chip, or systems in a package. Yet, integration of MEMS devices onto a single chip remains largely unrealized. The challenges of MEMS integration are very different from the challenges of CMOS (Complementary Metal Oxide Semiconductor). While CMOS technology has reached the 100 million transistor integrated circuit (IC) milestone, the world of MEMS is still mostly made up of discrete devices. The MEMS industry is highly fragmented, with little integration. Most MEMS are discrete devices, due to the demanding processes of conventional MEMS manufacturing.
Integration of MEMS devices, besides lowering manufacturing costs, will do much to expand the capabilities of the devices. Sensitivity of most microsensors falls off geometrically with decreasing size. For example, the output from a torsional capacitive accelerometer drops off as the fifth power of the lateral dimension.1 The problems of line capacitance and signal-to-noise ratio make it impractical to shrink such a sensor without onboard circuitry to detect and process such diminishing signals. In the industry today, the primary focus of MEMS sensor integration is to provide on-chip control circuitry. The automotive industry has been a leader in the combination of different sensors onto a single chip. For example, many pressure sensors and accelerometers address the temperature sensitivity of the response curves by adding on-chip thermometers for temperature compensation. Further steps have been achieved. For example, Toyota Nippondenso has reported an automotive sensor suite combining sensors for engine pressure and temperature, on to the same chip as an impact air bag trigger.2 Sensor suites, however, continue to be a niche market of highly specific functions and high volume products due to the complex design and extensive resources required to construct a suite. 1 Gabrielson, T. B., “Fundamental noise limits for miniature acoustic and vibration sensors”, Transactions of the ASME. Journal of Vibration and Acoustics, Vol. 17(4), p. 405 (1995).2 T. Fuji, Y. Gotoh, and S. Kuroyanagi, “Fabrication of Microdiaphragm Pressure Sensor Utilizing Micromachining,” Sensors and Actuators, A34:217 (1992).
Integration continues to be a difficult problem. Polysilicon is a core material for micromachining, and an excellent example of the conflicts in MEMS integration. The high temperatures of deposition (approximately 630° C.) and of annealing (>900° C.) are incompatible with aluminum and copper metalization. Either the process flow must be compromised, or a more expensive, and more resistive, refractory metal such as tungsten must be used.3 MEMS processing poses a unique set of challenges to integration resulting from the temperature-sensitive thin film materials, very deep etching, anodic bonding and strain-relief anneals that are required. Designing an integrated sensor suite on a single chip poses many challenges in combining the steps used to form common accelerometers with, for example, an IC temperature sensor or a thin film thermistor. Moreover, the resulting design will be inflexible; upgrading to an improved sensor requires a complete redesign and purchase of a new mask set. 3 K. A. Honer, “Surface micromachining techniques for integrated microsystems”, Ph.D. thesis, Stanford University, March, 2001.
Vertical integration, or stacking of microdevices into the same package, is an attractive way to decrease packaging volume, to increase circuit density and conserve board space, and to increase performance and functionality. Reductions of interchip delays and power consumption are both benefits of stacked integration. If the devices are thinned and stacked on top of each other, the advantages in cost and circuit density are potentially huge. For both IC and MEMS processes, the third dimension of the silicon wafer remains largely unexploited.
Current commercial approaches to vertical stacking of 2-dimensional devices are generally chip-scale and rely on wafer thinning by grinding. Most methods rely on interconnection by way of throughholes or wire-bonded, stacked mother-daughter chips. Current methods all have limitations with respect to package size, cost, reliability and yield impact. Despite the difficulties, stacking devices to achieve 3-D integration is finding applications, particularly in combining MEMS with ASIC (Application Specific Integrated Circuit) controllers. High density memory packages made by stacking individual chips have found specialty applications.
One implementation of 3-dimensional packaging has been undertaken by Irvine Sensors, Irvine, Calif., and IBM. Discrete die have been stacked and interconnected utilizing an edge lift-off process.4 Known-good-die (KGD) are thinned. Solder bumps at the die edge are used to align and interconnect the stacked die. The die are potted in an epoxy matrix. The epoxy helps to align different sized die, and is used as the interconnect surface. The individual stacking and interconnection of die, along with the requirement for KGD causes this to be a very expensive manufacturing method. 4 J. Minahan, A. Pepe, R. Some, and M. Suer, “The 3D stack in short form (memory chip packaging),” Proceedings 42nd Electronic Components and Technology Conference, San Diego, Calif., (1992)
Another implementation of 3-dimensional packaging has been undertaken by Cubic Memory, who manufactures high-density, stacked memory modules by applying gold interconnect traces that are deposited over insulating layers of polyimide on whole wafers. However, stacking and vertical interconnect is still on an individual chip-scale.
A further implementation of 3-dimensional packaging has been undertaken by Tessera, San Jose, Calif., in conjunction with Intel, to develop chip-scale, stacked package by attaching the chips onto flexible substrates via micro-ball grid array bonding, then z-folding the chip-loaded tape onto itself.
Ziptronix is apparently developing wafer-scale stacking of ICs. Considerable challenges with alignment, stress management, thermal management, high density interconnect and yield are still being addressed.
There are various deficiencies with available vertical integration. One primary deficiency is due to yield loss. All approaches to device stacking that are currently in the marketplace are die-scale. Individual die are prepared, aligned, stacked and connected. The processing is expensive and the yield loss for the stack is the compounded yield loss for each device in the layer. The increased yield loss is sometimes tolerated for inexpensive devices such as SRAM stacks. But when more expensive devices are being stacked, the solution is to use known good die (KGD). For KGD, each unpackaged die undergoes burn-in and test. Furthermore, the stack requires electrical test after the completion of each layer. The process is very expensive and the applications have been limited to high end users, such as military and satellite technology.
Another deficiency of conventional vertical integration is due to the fact that the technology is limited to a die-scale. With the exception of the yet-to-reach-the market approach of Ziptronix, all of the approaches to stacking devices are on die scale. The significant economic advantage of wafer-scale manufacturing is completely unavailable to these technologies. The high cost of handling and testing individual die restricts these methods to high-end applications.
Still another deficiency of conventional vertical integration relates to material incompatibility. Organic adhesive and potting compounds are used to build the stack. The use of adhesives and potting compounds is incompatible with many useful processes. The thermal coefficient of expansion (TCE) of the adhesive is generally not matched to the TCE of the wafers. Strict limits must be placed on the temperature and thermal cycling in subsequent processes, and in device operation to prevent die cracking and delamination. Moreover, most of the adhesives are organic compounds and thus are incompatible with semiconductor processes involving oxidizing ambient, high temperatures and aggressive chemical exposure.
Sensor integration remains a very expensive, design-intensive effort. Current sensor integration is primarily found in the automotive industry, the high design costs are amortized over the huge volume of parts manufactured. A new system and method of integration is needed to make the vast potential of integrated MEMS devices available to a broader applications.
Semiconductor and MEMS devices are made in only a small portion of the wafer thickness; the majority of the wafer thickness is for structural support during the manufacturing of the devices. Indeed, it is common to backgrind a finished wafer before packaging to improve the thermal transfer. An additional feature of very thin devices is that they are flexible, which is advantageous in managing the mechanical stresses of wire bonding and packaging. Despite advantages of very thin layers, thinning to less than 100 microns is very costly and therefore is seldom done. To avoid punching through in any region of the wafer, lapping must be performed at low rate and must be performed iteratively with careful wafer thickness mapping. Wafer thinning may be accomplished by wet etch or by plasma etch of the backside, with similar complexities with thickness uniformity and breakthrough. A layer may be incorporated in the wafer as an etch or polish stop. For example, a silicon nitride layer may be incorporated into silicon as a hard polish stop, or an implanted layer of born can stop a dopant-selective etch. While these methods are effective, they are costly and difficult to implement.
Applications for MEMS sensors have grown rapidly. The market size for all types of microsystems was estimated at over $14B year 2000, with a predicted 21% compounded annual growth rate (CAGR). Environmental monitors make up less than 5% of the market, but the forecasted CAGR of 35% over the next 4 years is much higher than average for the market.5 Improved cost and reliability are major drivers causing many conventional sensors to be replaced by microsensors. Microsensors are available to measure acceleration, vibration, pressure, temperature, humidity, strain, proximity, rotation, acoustic emission, and many others. Examples of applications include automotive air bag safety systems, other automotive applications, security systems, shock sensors, biomedical applications. 5 R. H. Grace, “The New MEMS and Their Killer Apps”, Sensors Magazine, July 2000.
Automotive air bag safety systems are triggered by MEMS accelerometers. Over 1,000 lives are saved every year thanks to air bag systems made affordable by MEMS sensors. The National Highway Traffic Safety Administration (NHTSA) estimates that hundreds more lives could be saved by smart air bag systems with a sensor array which adjusts for the severity and location of the impact, and for presence, position, motion and weight of the occupant.6 The sensor market for air bag deployment has enjoyed rapid growth of a 20%–25% CAGR over the preceding 5 years. 6 “Advanced air bags, final economic assessment”, FMVSS NO. 208, NHTSA Office of Regulatory Analysis & Evaluation, Plans and Policy, May, 2000.
Automotive applications for MEMS are enormous. MEMS sensors measure the level of engine oil, fuel, coolant, transmission and brake fluid. Pressure sensors monitor ABS line pressure, vacuum level, fuel injection pressure, tire pressure and more. Chemical and flow sensors are employed to monitor exhaust makeup, intake flows. Temperature sensors optimize engine performance, and along with humidity sensors, determine cabin comfort. Driver safety and convenience are enhanced by vehicle dynamic control for measuring yaw rate and by collision avoidance proximity sensors. There are many more. Cheaper and more powerful sensor suites have enormous potential to increase driver safety, improve cabin comfort, and to make engines longer lasting and more environmentally friendly.
Security systems combine sensor types to expand the net of detection and to limit false alarms through intelligent redundancy of alarms. Proximity, motion, vibration and heat detection are combined. Integrated sensor arrays have vast potential for battlefield sensor networks which monitoring troop strengths and movements. Miniaturized wireless communications integrated with microsensor suites will enable smart sensor webs with enormous potential.7 7 J. M. Kahn, R. H. Katz and K. S. J. Pister, “Mobile Networking for Smart Dust”, ACM/IEEE Intl. Conf. on Mobile Computing and Networking (MobiCom 99), Seattle, Wash., Aug. 17–19, 1999.
Shock sensors protect disk drives by inhibiting read/write operations during mechanical disturbances. Product lifetimes can be extended by data from vibration sensors, and imminent failure of critical components can be predicted, decreasing downtime of mission-critical systems. Environmental monitors hold great promise for product inventory and quality control monitoring, as well as water and air testing.
Biomedical applications are truly revolutionary, and go far beyond DNA sequencing to include new drug discovery techniques, as well as new and rapid testing for illnesses. Enormous improvements in quality of life will come from improved drug delivery methods biomechanical devices such as hearing aids and artificial vision.
Optical switches and optical switching components (e.g., variable optical attenuators) also are proposed and formed using MEMS, for example, including rotating micro-mirrors that direct light in desired directions, impart delays, and other functionality.
The microsystems market is big and getting bigger at a rapid rate. A method to build integrated sensor suites which is cost effective and universal to any type of sensor holds vast potential to create many new, exciting applications. Cheaper and more powerful sensors will have enormous positive impact on every aspect of society.
Various sensor technologies for MEMS temperature, humidity and shock sensors exist. Temperature can be measured by many means, the most common being Resistance Temperature Detector (RTD), thermistor and IC devices. It also possible to use capacitive measurement of pressure changes to generate an electrical signal based on a temperature change. This is generally implemented as a pressure sensitive oscillator, making power requirements relatively high. The RTD also requires a relatively high operating current, and self-heating can make short duty cycling difficult to implement. On the other hand, a very low power thin-film thermistor is straightforward to construct. An amorphous germanium thermistor has been reported which draws only 1 microA at 2V.8 The temperature coefficient of resistance (TCR) was reported as approximately −2%/K at room temperature. With such a low current drain, self-heating effect of the thermistor may be safely ignored. A further advantage is that the sensor can be operated with a power supply (battery) without requiring an external current source. Response curves, though parabolic, have a sufficiently linear characteristic that linearization may not be necessary. 8 G. Urban, A. Jachimowicz, H. Ernst, S. Seifert, J. Freund, F. Kohl, “Ultrasensitive Flow Sensors for Liquids Using Thermal Microsystems”, Eurosensors XIII, The 13th European Conference on Solid-State Transducers, p. 691(1999).
Integrated circuit sensors derive temperature from the known temperature dependence of the forward voltage of silicon junctions. CMOS thermometers operating at 3 V are commercially available. Low supply currents, well below 50 μA, generate very low self-heating—less than 0.1° C. National Semiconductor offers a low power CMOS thermometer which operates at 3V drawing<10 microA (NSC part no. LM19). Cost is $0.20 in quantities of <1000. Analog Devices manufactures a CMOS thermometer with a built-in shutdown function which cuts supply current to less than 0.5 μA.9. 9 Part no. TMP35/TMP36/TMP37, Analog Devices, Norwood, Mass.
Relative humidity sensors detect the change in a material property in response to absorption of atmospheric moisture. The material property of interest may be dielectric function as in capacitance gauges, electrical impedance in resistive humidity sensors, or thermal conductivity. Capacitive relative humidity (RH) sensors are simple devices used in many industrial and meteorological applications. Capacitive RH sensors have low temperature coefficients, and low power consumption (<10 microA).
Standard MEMS shock sensors are based on capacitive, piezoresistive, and piezoelectric measurements. An external electrical power source is required for variable capacitance sensors or bridge-type piezoresistive devices. However, piezoelectric (PE) generate an electrical signal without drawing current from an external electrical power supply. The high impedance output signal from a PE sensor makes detection susceptible to electromagnetic noise, and needs to be addressed in the measurement circuitry.
Many sensors also have on-board power, e.g., batteries. Of common, available batteries, lithium primary cells are have been used to meet extended battery lifetimes. Lithium batteries have 3V operating voltage and high energy density, long (>10 year) shelf life, good low temperature operation and excellent leakage resistance. They are also suitable for pulse discharge, should it be desired to duty cycle the sensor suite.
A long battery life requires a low average current drain. A low average current drain may be achieved by either extremely low constant drain current for always-on devices, or by duty cycling the sensor suite to higher operating currents by using a very low power clock relay. Energy densities for commercially available lithium coin cells range from 25–1700 mAh, and the capacity of the most typical lithium battery is 300–400 mAh. Considering a 400 mAh cell (Tadiran TL-5186), to approach a ten year battery lifetime, the average current drain must be less than 4.5 microamps. This is an extremely low operating current and outside the requirements for available accelerometers (shock sensors). Either a larger and more expensive cylindrical battery must be used (available up to 19 Ah), or the sensor suite must be triggered or duty cycled. While temperature and humidity are slowly changing variables suitable for sampling rates as low as hourly or less, an impact is a random event. A shock sensor must either be always on, or must be capable of rapid start-up after a trigger impulse. Package impacts are relatively short duration events (5–30 msec), thus a triggered impact sensor must be capable of sub-millisecond response from a sleep mode. Dallas Semiconductor DS1306E is a real time clock with alarm and operates with an average power drain of 1 microW, guaranteeing an overall sleep power dissipation of 1 microW.
Room-temperature drainage curves indicate that a ten year operational lifetime is possible for 3V lithium batteries if the drain current is <30 microamps.10 Continuous operation in very cold conditions (−21 C.) will reduce the lifetime by about one order of magnitude. 10 http://data.energizer.com/datasheets/_partof/splash.htm
To accommodate mass use of the aforementioned MEMS, and to integrate MEMS into many more aspects of everyday life, therefore, more economical manufacturing methods are needed. Manufacturing MEMS based on chip-scale techniques, or wafer scale techniques employing conventional thinning techniques, are not likely suitable for economical MEMS integration.